7-[2-(2-amino-4-thiazolyl) glyoxylamido] 3-(mercaptomethyl)-8-oxo-5-thia-1-azabicyclo-[4.2.0]oct-2-ene-2-carboxylic acid, of formula (Ia), generically known as ceftiofur
and its sodium salt of formula (Ib)
are valuable antibiotics for veterinary use, specially for treatment of infections in bovine animals.
The synthesis of ceftiofur (Ia) and cefiofur sodium (Ib) has been achieved the following ways:    1. U.S. Pat. No. 4,464,367, which covers ceftiofur and ceftiofur sodium, discloses two methods for preparation of ceftiofur, which comprises:            i) reaction of 7-amino-cephalosporanic acid (7-ACA) with (tritylamino-2-thiazolyl-4)-2-methoxyimino-2-acetic acid in presence of diccyclohexylcarbodiimide and hydroxy-1-benzotriazole, followed by reaction of the 7-acylated cephalosporanic acid thus obtained, with thiofuroic acid, followed by removal of the trityl protective group by treatment with trifluoroacetic acid to give ceftiofur (Ia) as summarized in Scheme-I        
This method is not only uneconomical but also poses serious safety hazards since it utilizes expensive and toxic dicyclohexylcarbodiimide for the coupling step and corrosive trifluoroacetic acid for removal of the amino protective group. Moreover, the by-product formed in the reaction, viz. dicyclohexyl urea is not easily removed requiring, more often than not tedious and elaborate purification methods.                ii) reaction of 7-[2-(2-amino-4-thiazolyl)-2-methoxyimino acetamido] cephalosporanic acid, generically known as cefotaxime of formula (IV) with thiofuroic acid to give ceftiofur (Ia) as summarized in Scheme-II.        
The conversion of cefotaxime to ceftiofur, which involves nucleophilic displacement of the acetoxy substituent (—OCOCH3) at 3α-position of the former compound with a thiofuroyl group (—S—CO—C4H3O) can be carried out in the presence of an acid or in the presence of a base
Substitution of an acetoxy function (—OCOCH3) by a thiofuroyl group (—S—CO—C4H3O) can be achieved through a SN1 or SN2 reaction but the success of these reactions strictly depends on various parameters. In the case of cephalosporin compounds, such a SN1 or SN2 reaction is highly dependent on the pH of the reaction i. e. whether it is carried out in the presence of an acid or a base, the medium of reaction, temperature of reaction etc.
However, apart from a cursory mention of the abovementioned reaction sequence summarized in Scheme-II, the U.S. Pat. No. 4,464,367 neither provides any detail whatsoever nor any enabling conditions for preparation of ceftiofur from cefotaxime, either through an example or description.
Whatever is mentioned is nothing but a conjecture with no evidence whatsoever to substantiate the conjecture, which would be evident from the discussion in the later part of this specification.


Further, the thiofuroyl substituent at 3α-position of the product i. e. ceftiofur is very vulnerable to acidic as well as basic conditions resulting in hydrolysis of furoyl group to give thiol (—SH) derivative at the 3α-position as well as formation of divalent sulfur derivatives. This makes it very difficult to effect a successful SN1 or SN2 substitution at the 3α-position of cefotaxime to obtain ceftiofur substantially free of the thiol compound and the divalent sulfur derivatives.
The problem is further compounded by the presence of the 2-[(2-amino thiazol-4-yl)-methoxyimino]acetamido addendum at the 7-position, which is also highly susceptible to cleavage under acidic and basic conditions to give the corresponding 7-deacylated derivative i. e. 7-free amino derivative.
The U.S. Pat. No. 4,464,367 further states that mineral salts of the corresponding acids are obtained by action on the free acid of formula (Ia) with a mineral base such as NaOH or KOH or NaHCO3 in equimolar quantity; the salification reaction is effected in a solvent such as water or ethanol and the salt obtained is isolated by evaporation of the solution.
However, there is no enabling disclosure in the patent specification for methods for preparation of ceftiofur sodium.    2. U.S. Pat. No. 4,937,330 describes a method for preparation of ceftiofur sodium of high purity comprising the steps of:            i) conversion of ceftiofur to a crystalline ceftiofur hydrohalide salt, specially the hydrochloride salt,        ii) neutralization of the crystalline hydrohalide salt of ceftiofur, specially the hydrochloride salt thus obtained by treatment with a basic resin eg. polyvinyl pyridine in an aqueous organic solvent,        iii) removal of the basic resin from the reaction mixture by filtration, and        iv) treating the filtrate containing the neutralised compound i. e. ceftiofur with a base of a sodium metal carrier to give ceftiofur sodium.        
However, this method is not only lengthy and less cost-effective since it involves the step of formation of a hydrochloride salt and its subsequent neutralization and utilization of expensive resins like polyvinyl pyridine.    3. U.S. Pat. No. 6,458,949 describes a process for preparation of ceftiofur sodium and its intermediates comprising reaction of silylated 7-amino-3-(2-furylcarbonylthiomethyl)-3-cephem-4-carboxylic acid (furaca) with 4-halo-2-methoxyimino-3-oxobutyryl halide to give an intermediate, which on cyclization with thiourea gives ceftiofur as summarized in Scheme-III.
However, this method involves a total of 6 steps, of which 4 steps are involved to prepare the 4-halo-2-methoxyimino-3-oxobutyryl halide intermediate, making it lengthy and tedious. Moreover, the yields reported are low rendering the process commercially not very attractive.
    4. Displacement of a functional group at of 3α-position of a cephalosporin derivative to give a 3-thiomethyl derivative is known in the art. Notable among such methods are:            i) the one disclosed in U.S. Pat. No. 4,312,986 for a process for producing a 7-(substituted)-amino-3-substituted thiomethyl-Δ3-cephem-4-carboxylic acid derivative of the formula (II) comprising,        
                 reaction of the compound of general formula (III),        
                 wherein R1 is a hydrogen atom or a C1-4 alkoxy group; R2 inter alia is essentially an amino group and X is a leaving group with a thiol compound of formula R8—SH, where R8 is a thiol compound residue, in an organic solvent in the presence of a Bronsted acid, or a Lewis acid or complex compound of Lewis acid other than BF3.        ii) the one disclosed in U.S. Pat. No. 6,476,220 for preparation of 7-amino-3-(2-furanylcarbonylthiomethyl)-3-cephem-4-carboxylic acid starting from 7-amino cephalosporanic acid and thiofuroic acid, by nucleophillic substitution of the acetoxy function at 3-position of the cephalosporin nucleus in (II) with a thiofuroyl function, as shown hereinbelow.        

This displacement reaction is carried out in the presence of boron trifluoride in ethyl acetate. Since boron trifluoride is a gas, and hazardous in nature, it requires special handling precautions on an industrial scale.
However, the above patents essentially teach the functionalisation at 3-position of compounds of formula (II) and 7-ACA, which have a free amino group at 7-position of the cephalosporin nucleus. There is, however, no suggestion that 7-acylamino cephalosporins, specially those carrying a 2-aminothiazoylyl acetamido moiety at the 7-position could be reacted with a thiol compound, R8—SH, in the presence of Bronsted acids or Lewis acids, to give the corresponding 7-(2-aminothiazolyl)-acetamido-3-substituted thiol derivative.
Further, Bronsted acids or Lewis acids are known to effect the cleavage of the amide bond at 7-position of cephalosporins. Incidentally, U.S. Pat. No. 5,132,419 relates to a process for preparation of 7-amino-3-[(Z)-1-propen-1-yl]-3-cephem-4-carboxylic acid of formula (V), by reaction of the corresponding 7-acyl amino derivative of formula (IV), wherein the cleavage of the amide bond at 7-position is effected by utilization of a Bronsted acid or Lewis acid. The chemistry is summarized hereibelow,
wherein R1 and R2 represents alkyl, aryl and aralkyl radicals, either identical or different.
Incidentally, the above fact was substantiated by the present inventors when in their attempts to convert cefotaxime to ceftiofur in the presence of a Lewis acid like Boron trifluoride etherate, the 7-deacylated derivative of cefotaxime i. e. 7-ACA was the major product, with very little conversion to ceftiofur observed.
From the foregoing, it is abundantly clear that:    a) while displacement of a functional group at of 3α-position of a cephalosporin derivative by reaction with a thiol compound in the presence of a Bronsted acid or a Lewis acid to give the corresponding 3-thiomethyl derivative is achieved, when the 7-amino function is unsubstituted i. e. is free;    b) the corresponding displacement of a functional group at of 3α-position of a cephalosporin derivative, wherein the 7-amino function is substituted i. e. carrying an acylamino function by reaction with a thiol compound in the presence of a Bronsted acid or a Lewis acid would be expected to lead to substantial cleavage of the 7-acylamino function or the amide bond to give the respective starting compounds with a free 7-amino group, with little or no displacement reaction taking place at the 3α-position.
Against this backdrop, reaction of compound of formula ((III), wherein the 7-amino function is substituted, specially with a 2-aminothiazolyl acetic acid moiety with the thiol compound, R8—SH in the presence of a Bronsted acid or a Lewis acid would be expected to cleave the amide bond at 7-position leading to formation of the deacylated products, with little or no displacement reaction taking place at the 3α-position.
Indeed, the present inventors found it to be true when 7-[2-(2-amino-4-thiazolyl)-2-methoxyimino acetamido] cephalosporanic acid i.e. cefotaxime or its salts or its easily hydrolysable esters of formula (VI),
wherein R3 is hydrogen, an alkali or alkaline earth metal, or an easily hydrolysable ester, is reacted with thiofuroic acid, in the presence of a Lewis acid such as aluminium chloride; boron trifluoride; zinc halides such as zinc chloride and zinc bromide; tin halides such as stannic chloride and stannic bromide; or the complex compounds of these Lewis acids with dialkyl ethers, amines, fatty acids, nitriles, carboxylic esters and phenols etc. very little displacement at the 3α-position took place and the reaction led to formation of predominant amounts of the 7-deacylated derivative i. e. 7-ACA, and unidentified impurities. Ceftiofur of formula (I) formed in the reaction was only between 5-15% and could not be isolated from the reaction mixture.
Similarly, when instead of a Lewis acid, the abovementioned reaction was carried out in the presence of a Bronsted acid like hydrochloric acid, p-toluenesulfonic acid, benzenesulfonic acid, trifluoromethane sulfonic acid, naphthalene sulfonic acid, super acids such as perchloric acid, magic acid (FSO3H—SbF5), FSO3H—AsF5, CF3SO3—H—SbF5, H2SO4—SO3, chloro sulfuric acid, fluoro sulfuric acid, etc. the same phenomena was observed i. e. very little displacement at the 3α-position took place and the reaction led to formation of predominant amounts of the 7-deacylated derivative i. e. 7-ACA, and unidentified impurities. Ceftiofur of formula (I) formed in the reaction was only between 5-20% and could not be isolated from the reaction mixture.
Further, carrying out the abovementioned reaction in the presence of a mixture of Bronsted acid and a Lewis acid was also found to result in predominant amounts of the 7-deacylated derivative i. e. 7-ACA, and impurities and here again whatever ceftiofur formed in the reaction could not be isolated from the reaction mixture.
The above conversion, hence, has no commercial application, except may be of some academic interest.
Against this backdrop, it was with surprising effect that the present inventors found that the reaction of 7-[2-(2-amino-4-thiazolyl)-2-methoxyimino acetamido] cephalosporanic acid i.e. cefotaxime or its salts or its easily hydrolysable esters of formula (VI), wherein R3 is hydrogen, an alkali or alkaline earth metal, or an easily hydrolysable ester,
with thiofuroic acid can be carried out selectively to give a conversion to ceftiofur of formula (Ia),
between 40% to 70% when the said reaction is carried out in the presence of either methanesulfonic acid or sulfuric acid.
However, again to their surprise the present inventors found that while the reaction of ceftotaxime with thiofuroic acid in the presence of sulfuric acid indicated a conversion of between 40 to 55% to ceftiofur, and formation of impurities in the range of between 8 to 40%, however, no product could be isolated from the reaction mixture, in spite of employing various isolation methods.
A dramatic change was found when methanesulfonic acid was used as the Bronsted acid instead of sulfuric acid. Not only a conversion of between 50 to 70% was achieved, but most importantly, ceftiofur could be indeed be isolated from the reaction mixture in substantially large amounts and surprisingly the isolated product was found to possess high stability, high purity of more than 97% and found to be substantially free of impurities.
More surprising was the finding that though methanesulfonic acid gives a commercially viable conversion of cefotaxime to ceftiofur conforming to pharmacopoeia specification, other organic sulfonic acids like benzenesulfonic acid, p-toluenesulfonic acid, trifluoromethanesulfonic acid etc. failed miserably to give a viable process.
The yield and quality of ceftiofur and formation of impurities in the abovementioned reaction was also found with surprising effect to be highly dependent on the amount of methanesulfonic acid employed, the amount of thiofuroic acid employed, the medium and temperature of reaction. The selection of the molar proportions of methanesulfonic acid, the medium of reaction and the temperature of reaction in providing ceftiofur (Ia) in large isolable yields and high purity, therefore forms the basis of the present invention.
It was found that ceftiofur of formula (Ia) and ceftiofur sodium of formula (Ib) could be obtained in isolable yields of between 20% to 25% possessing high storage stability and having purity more than 97% and substantially free of impurities through a selection of parameters in the reaction of 7-[2-(2-amino-4-thiazolyl)-2-methoxyimino acetamido] cephalosporanic acid i.e. cefotaxime or its salts or its easily hydrolysable esters of formula (VI) with thiofuroic acid, the selection being utilization of:    i) not a catalytic amount but a large excess of methanesulfonic acid in molar proportions of 12-18 moles per mole of compound of formula (VI),    ii) an excess of thiofuroic acid in molar proportions of 1.5 to 3.0 moles per mole of compound of formula (VI),    iii) specifically acetonitrile as the medium of reaction, and    iv) a reaction temperature in the range of between −5° C. to 30° C., preferably between 10° C. to 30° C., and more preferably between 15° C. to 25° C.
It was further found that the ceftiofur (Ia) thus obtained could be converted to tis sodium salt i. e. ceftiofur sodium of formula (Ib), again possessing high storage stability and having purity more than 97% and substantially free of impurities through a selective method comprising double decomposition of a salt of ceftiofur with an organic amine with a sodium metal carrier and isolation of ceftiofur sodium from the reaction mixture through a solvent precipitation method.
Preparation of ceftiofur sodium (Ib) of high storage stability and having purity more than 97% and substantially free of impurities from cefotaxime (VI), via the intermediacy of ceftiofur (Ia), forms the basis of another aspect of the present invention.
The ceftiofur (Ia) and its sodium salt (Ib), thus obtained by virtue of their high purity and stability are found to be highly suitable for formulation into suitable dosage forms.